Study of the mechanisms of tolerance to primarily vascularized renal allografts in miniature swine

Transplantation of a vascularized allograft in a host leads to acute rejection of the graft and elimination of the foreign antigen. This rejection process can be avoided by inhibition of the immune response. There are different ways in which one can overcome the immune response to an allotransplant: matching, nonspecific immunosuppression and specific transplantation tolerance. The matching is based on the fact that allotransplants are rejected because the recipient’s immune system recognizes on donor cells histocompatible antigens that are different from his own. If there are no such disparate antigens, such as in identical twins, then no rejection response occurs. The most important histocompatibility antigens are those of the Major Histocompatibility Complex (MHC); known in human beings as the Human Leukocyte Antigen (HLA) system. However, there are numerous other antigens capable of causing rejection, termed minor histocompatibility antigens. Matching to avoid or eliminate the immune response in conceivable in kidney or bone marrow transplants, since the time necessary to carry out tissue typing is available. Matching, however, is unrealistic in case of liver, heart or lung transplants, since the admissible ischemia time does not allow the achievement of tissue typing.
The use of nonspecific immunosuppressive drugs represents the most practical means to avoid rejection. This therapy is accountable for the enormous success in the field of transplantation over the past three decades. Such a drug regimen, however, can produce major complications during the early as well as long-term follow-up such as infections, malignancies or drug-induced side-effects. Long term, the immunosuppressive regimen therefore may directly cause loss of patients ad cannot prevent chronic rejection, which is the second major cause of graft loss.
Therefore, induction of specific transplantation tolerance still represents a major goal of modern transplantation immunology. Specific transplantation tolerance refers to the elimination of the immune response to the antigens of the transplant, while the immune response to all other antigens in the environment remains intact. Thus, although nonspecific immunosuppressive drugs eliminate immune cell populations with a broad range of reactivities, the induction of specific tolerance leads to loss or inactivation of only those immune cell populations with receptors specific for the antigens presents by the transplant. In the early 1950s, such tolerance has been demonstrated by Billingham and Medawar, who showed that injection of bone marrow in neonatal mice across MHC barriers was capable of producing tolerance to the tissue grafts from the bone marrow donor strain. Thus, an immature immune system such as in a fetus, or even on a neonate, is capable of being modulated. This concept has been successfully applies to human beings, (in utero or neonate), using stem cells or fetal liver cells in case of severe combined immunodeficiencies or inborn errors of metabolism. However, exposure of mature T-cells, to alloantigens generally leads to an immune response rather than to tolerance, and in order to induce similar transplantation tolerance in adult humans whose immune system has matured, other methods are needed. This is the goal of research on the field of specific transplantation tolerance, and many approaches have already been studied experimentally. Generally, one considers tolerance occurring through negative selection in the thymes as “centre tolerance” and tolerance occurring outsides the thymus as “peripheral tolerance”. There is therefore a natural tendency to consider clonal deletion of immature T-cells as the predominant mechanism of central tolerance and anergy/suppression of mature T-cells as the predominant mechanisms of peripheral tolerance. This is undoubtedly an oversimplification, since continuous interactions between peripheral events and those occurring in the thymus could have to occur in order to maintain a steady immunologic state. Intrathymic recognition of self antigens is thought to produce deletion of high affinity T-cell clones at an early stage of differentiation and thereby to be responsible for self tolerance. A similar mechanism can be envisioned for the development of clonal deletion to alloantigens, assuming that appropriate cells bearing those antigens, probably dendritics cells, are capable of homing to the thymus. Clearly, such homing occurs after bone marrow transplantation, and the possibility exists that it also occurs after aolid organ transplantation. There is increasing evidence that under some conditions, mature T-cells can also be tolerized, possibly by clonal deletion, as the result of a powerful immune response, but more likely as the consequence of the interaction of such T-cells with inappropriate or “non-professional” antigen-presenting cells. In the later situation, termed anergy, the T-cells are apparently paralyzed by the encounter of the antigen without co-stimulation, and are rendered unresponsive to MHC-antigens presented . Finally, the concept of induction of unresponsiveness by specific suppressor cells has recently fallen our of favour among immunologists because of failure to identify specific clones with appropriate activity. However, there is abundant data in the literature indicating that specific suppression of immune responses does occur, and, whether due to specific cells or to network interactions among a variety of cells, the possibility that such suppression may account for induction of transplantation tolerance must also be considered.
Induction of tolerance to primarily vascularized allografts has been reported mainly in rodents and occasionally in large animals by infusions of donor, mature or immature, lymphoid cells prior to transplant, or by using transient therapy with several kinds of specific (anti-CD4, anti-CD8, anti-CD2 monoclonal antibodies) or nonspecific immunosuppressive drugs such as anti-thymocyte Globulin or Cyclosporine A. Induction of indefinite and specific transplantation tolerance to primarily vascularized rend will be the subject of this thesis